1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording medium, such as electrophotographic copying apparatus, laser beam printer (LBP), facsimile apparatus, word processor or the like.
2. Related Background Art
In an image forming apparatus in which plural developing units respectively containing toners of different colors are positioned along a running image bearing member and are activated in succession to form a color toner image, it is conceived to equip the developing units respectively with solenoid clutches and to activate said developing units in succession by on-off operations of said solenoid clutches. In such operating system, there is required a solenoid clutch for each developing unit. Thus, in case of forming a full color image with toners of four colors of cyan, magenta, yellow and black, there are required four developing units, or namely four solenoid clutches. Since such solenoid clutches are expensive, such system leads to an increased cost of the entire apparatus.
Also since the solenoid clutches are relatively bulky, there is required a large space for the installation thereof.
On the other hand, in the conventional technology, since the developing units are positioned inside the apparatus, it is difficult for the operator to judge whether the selected developing unit is properly functioning.
Therefore, in case of eventual defect in the image, it is difficult for the operator to judge whether such defect is induced by the improper function of the developing units.
In the conventional technology employing mechanical switch means composed of gears and cams for transmitting the driving force, a shift gear has to be moved between a released position and an engaging position by cam member. However, if the cam groove of the cam member is displaced from the desired position of the shift gear, it cannot move to the engaging position so that the latent image formed on the photosensitive drum cannot be developed in a visible toner image by a desired developing unit. In such case, the operator can know the absence of transfer of a toner image of a certain color only by observing the recording sheet discharged from the apparatus, and has to repeat the image forming operation from the beginning. This fact results in a significantly deteriorated work efficiency.